1. The Field of the Invention
The invention relates generally to computers hosting interpreted languages and emulators, and more specifically to accelerators for emulators and interpreters such as Windows.TM., JAVA, Visual Basic, and other virtual machine environments executable by processors having access to caches.
2. The Background Art
Operations executed by a processor of a computer proceed in a synchronization dictated by a system clock. Accordingly one characteristic of a processor is a clock speed. For example, a clock speed may be 33 megahertz, indicating that 33 million cycles per second occur in the controlling clock.
A processor may execute one instruction per clock cycle, less than one instruction per clock cycle, or more than one instruction per clock cycle. Multiple execution units, such as are contained in a Pentium.TM. processor, may be operated simultaneously. Accordingly, this simultaneous operation of multiple execution units, arithmetic logic units (ALU), may provide more than a single instruction execution during a single clock cycle.
In general, processing proceeds according to a clock's speed. Operations occur only as the clock advances from cycle to cycle. That is, operations occur as the clock cycles. In any computer, any number of processors may exist. Each processor may have its own clock. Thus, an arithmetic logic unit (ALU) may have a clock operating at one speed, while a bus interface unit may operate at another speed. Likewise, a bus itself may have a bus controller that operates at its own clock speed.
Whenever any operation occurs, a request for interaction is made by an element of a computer. Then, a transfer of information, setup of input/output devices, and setup of the state of any interfacing devices, must all occur.
Each controller of any hardware must operate within the speed or at the speed dictated by its clock. Thus, clock speed of a central processing unit does not dictate the speed of any operation of a device not totally controlled by that processor.
These devices must all interface with one another. The slowest speed will limit the performance of all interfacing elements. Moreover, each device must be placed in the state required to comply with a request passed between elements. Any device that requires another device to wait while some higher priority activity occurs, may delay an entire process.
For example, a request for an instruction or data within a hard drive, or even a main, random-access memory, associated with a computer, must negotiate across a main system bus. A central processing unit has a clock operating at one speed. The bus has a controller with a clock that may operate at another speed. The memory device has a memory management unit that may operate at another speed.
Further to the example, a Pentium.TM. processor having a clock speed of 100 megahertz may be connected to peripheral devices or main memory by an industry standard architecture (ISA) bus. The ISA bus has a specified clock speed of 8 megahertz. Thus, any time the Pentium.TM. processor operating at 100 megahertz requests data from the memory device, the request passes to the opposite side of the ISA bus. The data may not be processed or delivered at a speed greater than that of the bus at 8 megahertz. Moreover, a bus typically gives low priority to the central processing unit. In order to avoid underruns and overruns, the input/output devices receive priority over the processor. Thus, the 100 megahertz processor may be "put on hold" by the bus while other peripheral devices have their requests filled.
Any time a processor must access any device beyond its own hardware pins, the hardware interface to the computer outside the processor proper, the required task cannot be accomplished within one clock count of the processor. As a practical matter, a task is not usually completed in less than several clock counts of the processor. Due to other priorities and the speeds of other devices, as well as the need to adjust or obtain the state configurations of interfacing devices, many clock counts of a processor may occur before a task is completed as required.
Associated with every hardware interface between hardware components, elements, and the like (anything outside an individual integrated chip), a hardware handshake must occur for any communication. A handshake, including a request and an acknowledgement, must occur in addition to a transfer of actual data or signals. Handshake protocols may actually involve several, even many, clock counts for the request alone, the acknowledgement alone, and for passing the data itself. Moreover, a transmission may be interrupted by a transaction having a higher priority. Thus, communicating over hardware interfaces is relatively time consuming for any processor. Hardware interfacing may greatly reduce or eliminate the benefits of a high-speed processor.
To alleviate the need to communicate across hardware interfaces during routine processing, modern computer architectures have included caches. In general, processors benefit from maintaining as close to themselves as possible all instructions, data, and clock control. This proximity reduces the need for interfaces, the number of interfaces, the interface complexity, and thus, the time required for compliance with any instruction or necessary execution. Thus, caches have been moved closer and closer to the processor.
Memory caches are common. Such a cache is created within a dedicated portion of a memory device. These are different, however, from caches dedicated to a processor.
The INTEL 386.TM. processor contains an optional external cache connected to the processor through a cache controller chip. The INTEL 486.TM. contains an internal 8 kilobyte cache on the central processing unit itself. Within the chip containing the processor, is integrated a cache. This cache is dedicated to both code and data accesses.
The 486.TM. also supports another cache (a level-2 cache, as opposed to the primary or level-1 cache just described above). Access to the level-2 cache is through an external cache controller chip, similar to that of the 386.TM.. In each case, for both the 386.TM. and 486.TM. processors, the external cache controller is itself positioned on a side of the processor's internal bus (CPU bus) opposite that of the processor.
The Pentium.TM. processors contain a level-1 (primary) data cache as well as a level-1 code cache. Thus, code and data are segregated, cached separately. The Pentium.TM. processors continue to support an external, level-2 cache across a CPU bus.
One should understand that the expression "bus", hereinabove, refers to the processor bus, rather than the system bus. For example, the main system bus connects a processor to the main memory. However, the cache controllers and caches on a processor, or external to the processor but simply located across a processor's internal bus interface unit, do not rely on the main system bus.
A cache has some fixed amount of memory. A code cache will contain certain executable instructions, a data cache will contain data, and a non-segregated cache may contain both. The memory of any type of cache is typically subdivided into cache lines. For example, a typical cache line may contain 32 bytes of information. Thus, a cache line contains a standard number of bytes in which space may be stored a copy of certain information obtained from a main memory device.
Associated with each cache line is a tag. The tag binds a physical address and a logical address corresponding to the contents of an associated cache line.
The physical and logical addresses contained in the tag associated with a cache line may correspond to a physical location in the main memory device, and a logical position within an application respectively.
Caches associated with a processor are transparent, even hidden, with respect to a user and an application. Each cache has an associated controller. In operation, a cache controller effectively "short circuits" a request from a processor to a memory unit. That is, if a particular address is referenced, and that address exists in a tag associated with the contents of a cache line in a cache, the cache controller will fulfill the request for the instruction out of the cache line containing it. The request is thus fulfilled transparently to the processor. However, the effect of a cache is to eliminate, as much as possible, communication through hardware interfaces as described above. Thus, a cache may greatly improve the processing speed of applications running on processors.
Tags may also have associated therewith two numbers referred to as "use bits." The use bits may typically represent a simple count of use. This count may be useful to the cache controller in determining which cache lines are the least recently used. Accordingly, a cache controller may refer to the least recently used (LRU) count to determine which cache lines have been referenced the least number of times.
Incidently, but significantly, with respect to the invention, some cache controllers may churn a cache. That is, if an insignificant number of bits is contained in the LRU or use bits, then a counter may be improperly reset to zero due to count "wrap-around" during high use. Thus, highly-used cache lines may actually be swapped out, churning the cache and dramatically decreasing efficiency.
Several difficulties exist with caches. A cache controller has a general purpose function to service address requests generally. For example, a virtual machine may be implemented in some limited number of instructions. In operating such a virtual machine, a computer processor has an underlying native language in which the virtual machine instructions are written. The virtual machine instructions will be requested repeatedly. The virtual machine instructions are accessed relatively slowly if they are treated simply as another general purpose instruction being retrieved periodically into the cache.
Many processors pipeline instructions. Two problems may occur with pipelining. The first is flushing a pipeline as a result of a branch. The other is stalling due to requested data not arriving within a next clock count in sequence. That is, whenever a cache "miss" occurs, a request has been made to the cache, but the cache cannot respond because the information is not resident. Misses may occur repeatedly over extensive numbers of clock counts while a cache controller accesses a main memory device to load the requested instructions or data. Misses decimate the efficiency of processors. Meanwhile, even with branch prediction methods, a pipeline may flush several instructions with a resulting loss of processing performance.